1848.] Construction of Iron Tension Bridges. 415 



sion Bridges are peculiarly liable, some other arrangement of the same 

 quantity of Metal, as is now given to bridges on the uniform chain 

 system, may not with advantage be employed. 



6. Here it will not be irrelevant to observe that all the expedients 

 had recourse to, for the purpose of counteracting the vibration and 

 undulation of the uniform chain bridges, not only, of course, increase 

 the expense, and weight of the structure, but absolutely negative the 

 principal advantage expected from, and claimed for, that system, (viz* 

 the simplicity and directness of the strains,) in the ratio of their attain- 

 ing the object for which they were added, i. e. the stiffness of the 

 whole. 



7. Before proceeding to show, and I trust to prove, what will be a 

 more advantageous disposition of a given weight of metal in a bridge 

 of known size and proportions, than that which would be attained by 

 the uniform chain principle, it will be necessary to notice a mode of 

 construction for which a patent has been obtained by Mr. Dredge, who 

 proposes to erect bridges of equal, or even greater strength, than those- 

 on the uniform principle, with about -Jd of the quantity of iron usually 

 employed in the latter ; but as the practicability of such a result is 

 wholly at variance with the demonstration proved by the calculations 

 of the Rev. Mr. Pratt, now under reference, and as no one has yet 

 impugned the correctness of the formulae on which the strength of the 

 uniform chain system is calculated, it is scarcely necessary to do more 

 than base the rejection of Mr. Dredge's extreme taper chain system on 

 the grounds of its non-conformity with the rules quoted above ; unfor- 

 tunately however, the Bailee Khal Bridge near Calcutta, originally 

 constructed in strict accordance with this principle, which fell by its 

 own weight, and the inability of the " Kubudduk" Bridge near Jessore 

 in Bengal, to withstand the ordinary proof trial, together with its subse- 

 quent failure, sufficiently confirm the accuracy of Mr. Pratt's conclu- 

 sions. The iron work of the latter bridge was constructed by Mr. 

 Dredge himself. 



8. In the beginning of this "Paper" I remarked that I had 

 practically, i. e. experimentally corroborated the fact demonstrated in 

 Mr. Pratt's Memoir* and the failure of the Bailee Khal Bridge led to 

 so much study and research into the principles which should govern a 



* Vide account of " Experiments" at the end of this Memoir. 



